1. Field of the Invention
The present invention relates to a hybrid construction machine, and in particular, to a hybrid construction machine whose upper swing structure is driven with a hydraulic motor and an electric motor.
2. Description of the Related Art
Hydraulic actuators have been used widely in the field of construction machines since devices in the construction machine can be implemented in small sizes and light weights relative to the outputs of the devices. In recent years, however, construction machines equipped with electric actuators for increasing the energy efficiency are being proposed and some of such construction machines are already in practical use today. Especially, a swing actuator for driving and swinging the upper swing structure of a construction machine with respect to the lower travel structure is a rotary actuator of a high inertial load that is used (started and stopped) frequently. Using an electric actuator as the swing actuator is highly advantageous for improving the energy efficiency since energy recovery (energy regeneration) can be expected when the upper swing structure is decelerated or stopped.
In the situation described above, construction machines driving the upper swing structure using a hydraulic motor and an electric motor together have become known as hydraulic construction machines capable of conducting the energy recovery with high efficiency, as described in Japanese Patent No. 4024120, JP,A 2005-290882 and JP,A 2008-63888.